Technical Field
This disclosure relates to filters and duplexers that use acoustic resonators or components in cancellation architectures to achieve good performance for various communication systems and standards.
Description of Related Art
Radio frequency (RF) filters and duplexers have been essential components of communication systems. High selectivity, low insertion loss, compact size, ability to handle large signals (power handling), high linearity, manufacturability, and low cost may be some of the important desired features for RF filters and duplexers.
The requirements for RF filters and duplexers have become more stringent in light of new communication standards where information channels and frequency bands are closer to each other, new communication devices such as smartphones where the footprint and cost of all components must be very small as more components are needed in support of multiple standards and applications, and co-existent communication systems where multiple communication transmitters and receivers work simultaneously.
Linearity, noise, and power handling requirements typically lead to utilization of passive RF filters and duplexers in many applications. The performance of passive RF filters and duplexers may be limited by the quality factor (Q) of the components that are used in their realization. The filter selectivity as well as passband requirement may lead to a filter topology and filter order. For a given RF filter or duplexer topology and order, insertion loss may reduce with the increase of component Q.
Various technologies can be used to realize passive RF filters and duplexers. For instance, capacitors, inductors, or transmission lines can be used to realize passive RF filters and duplexers. Electromagnetic resonators, including waveguide resonators and dielectric resonators, can also be used to realize passive filters and duplexers. The quality factor of such components is typically proportional to their overall physical size. As such, it has been difficult to realize compact low-loss selective passive RF filters and duplexers using electromagnetic components and resonators.
Piezoelectric material can be used to realize compact high-Q resonators. Crystal resonators have been widely used to generate spectrally-pure oscillators. Surface acoustic wave (SAW) resonators have been widely used to realize compact low-loss selective RF filters and duplexers as well as oscillators. More recently, bulk acoustic wave (BAW) resonators have been used to construct high-performance RF filters and duplexers as well as oscillators. Ceramic resonators and micro electro mechanical system (MEMS) resonators with high quality factor have also been used in frequency generation as well as filtering applications.
RF SAW filters and duplexers have been used widely in wireless communications such as cellular phones, wireless local area network (WLAN) transceivers, global positioning system (GPS) receivers, cordless phones, and so forth. RF SAW filters have been used as band-select filters, image-reject filters, intermediate frequency (IF) filters, transmitter noise or spur reduction filters, and so forth. A typical smartphone may have several SAW resonators, SAW filters, and SAW duplexers to support various communication systems and standards.
Over the past decade, significant research and development on BAW technology has resulted in BAW resonators that have lower loss (or higher Q) or are more compact, especially at higher frequencies, compared with SAW resonators. Therefore, RF filters and duplexers that use BAW resonators may have lower insertion loss, or higher selectivity, or smaller form factor compared with those that utilize SAW resonators especially at higher frequencies. Thin film bulk acoustic resonators (FBAR) are a common example of BAW resonators.
Modern wireless communication standards designate many different operational frequency bands to support the increase in the overall wireless capacity and reach. For instance, cellular phone standards may include RF frequency bands that span around 700 MHz to around 4000 MHz. Furthermore, in order to increase the overall wireless capacity, the frequency spacing between adjacent frequency bands or channels within the same application or different applications may be reduced. This may be done, for instance, by reducing the typical guard bands in wireless standard or by placing the transmit and receive frequency bands in a frequency division duplex (FDD) scheme closer to each other. As a result, RF filters and duplexers with higher selectivity may be required. More selective RF filters and duplexers that utilize a given component or technology (SAW, BAW, etc.) may incur more in-band insertion loss. The higher RF filter or duplexer insertion loss may reduce the wireless receiver noise figure and sensitivity, increase the wireless transmitter power consumption or reduce the transmitted power, and/or deteriorate the overall performance of a communication system.
In commercial systems, the choice of technology may depend on the technical performance, such as power consumption as well as economic and business considerations such as cost, size, and time to market. For instance, while one technology may offer a better performance compared with another technology, it may not be adopted for a commercial system that is cost sensitive. In the case of RF filters and duplexers, it may be desirable to use a technology that leads to the lowest-cost and/or most-compact solution, as long as a predetermined performance criterion is met. In other words, a more expensive or larger solution may not be adopted, even if it offers a better performance as compared with an alternative solution that meets an acceptable performance level at a lower cost and/or size. For instance, while RF filters and duplexers that use BAW resonators may offer lower loss compared with RF filters and duplexers that use SAW resonators for a given set of specifications, the higher relative cost of BAW technology, as well as its relatively smaller number of suppliers, may disfavor their usage in certain applications and standards. Other considerations may be the ease of integration with the rest of the components in a communication system. For instance, there may be performance, business, or economic advantages to integrate RF filters and duplexers with low noise amplifiers (LNA), power amplifiers (PA), transmit/receive (T/R) or band-select switches, impedance matching networks, etc.
It may be desirable to realize selective low-loss compact passive RF filters and duplexers using technologies that are low cost and widely available. As stated before, given a filter topology and filter order, technologies that offer lower quality factor components may result in higher in band insertion loss. However, the filter topology can be modified to meet the desired set of specifications, without compromising the in-band insertion loss. For instance, a known unwanted out-of-band signal may be removed or reduced by using a band stop or notch filter, without necessarily increasing the order of an RF band pass filter (BPF). The source of this unwanted out-of-band signal may be a received interferers or blocker, jammer, transmitter in an FDD scheme, other co-existent communication devices. For instance, a WiFi transmitter can create unwanted signal for a cell phone receiver that is operating at a close-by frequency band within the same platform (smartphone). The undesired out-of-band signals may also be removed or reduced by using a feed-forward cancellation scheme in which an identical copy of the undesired signal is recreated and subtracted at the desired location.
As was mentioned previously, modern wireless communication standards designate many different operational frequency bands. Consequently, system complexity may significantly increase, since each of the frequency bands may require use of at least one RF filter. It may be desirable to reduce the complexity of such RF systems. One consideration is the use of a tunable RF filter that can replace many fixed frequency RF filters. However, conventional acoustic-based filters, such as SAW and BAW, may be not be tunable. Furthermore, tunable filters may be presented by combining acoustic resonators and tunable electromagnetic components, such as capacitors. However, these may still suffer from higher in-band insertion loss, lower rejection, lower isolation in case of a duplexer, or all of the above, compared to a modern commercial radio front-end system.